The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures.
Keywords: Photocharging; carrier trapping; colloidal quantum dots; electron spin; ultrafast transient spectroscopy.